[Q] Default Notification Sound, but no app shows

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So I have an odd issue ... there is some app( presumebly ) that is sounding my default notification sound at ( seemingly ) random intervals, however no app shows up in the notification center. I have no idea how to track down what app, or what notification is doing this.

Has anyone seen this, or know what could sound off without actually showing you the notification ... I mostly only have notifications on for Gmail, Voicemail, GoSms. I have several other apps installed ( Foursquare, Facebook, and maybe a couple others ) that are capable of notifications, but their all turned off ... and even if they were sounding, wouldn't you think I would see something in the notification center dropdown???

This is getting -extremely- annoying ... any help what-so-ever would be appreciated...